Representative, for instance, of some kinds of development activities occurring in the past 10 years for reducing the weight of automobiles and thereby enabling the autoist to be more conservative of fuel, would appear to be the following.
According to the May 1969 issue of Autoproducts, the G.R.T.L. Company, a joint development venture of PPG Industries, Inc. and Union Carbide Corporation, was formed (about 1968) to produce a family of glass-reinforced thermoplastic sheets called "Azdel" which could be formed on conventional metal-stamping apparatus for the purpose of meeting the automobile industry's need for high-speed productivity, for example, 180 to 360 parts per hour with a single press. The Azdel glass-reinforced thermoplastic sheet can be formed in one operation into shapes that take four or more separate stamping operations when working with a sheet of steel. The Azdel sheet is preheated in an infrared oven to about 400.degree. F. (about 204.degree. C.), then fed to a press and formed between cooled matched metal dies. The operation, according to the article, is scrap-free and as the stamped Azdel sheet comes from the mold it has no flash or trim, and holes and notches can be formed in the stamping operation. Shapes can be stamped from Azdel sheets that would be impossible in steel; and in some cases an assembly of several parts in steel can be redesigned so that it can be made in one part from an Azdel sheet.
Also according to the Jan. 22, 1968 issue of Chemical and Engineering News, the Azdel glass-reinforced thermoplastic sheets reportedly could be made from styrene acrylonitrile copolymer (42% glass reinforced), polyvinyl chloride (36% glass reinforced), polypropylene (44% glass reinforced) or other resins. The Azdel sheet contains generally about 40% glass fiber by weight. Reinforced polypropylene has a heat distortion temperature of 327.degree. F. (about 164.degree. C.); styrene acrylonitrile copolymer, 255.degree. F. (about 124.degree. C.); and polyvinylchloride, 221.degree. F. (about 105.degree. C.). A given part can be formed from a variety of blank sizes, such as an 84-mil-thick hood can be formed from a 150-mil-thick by 100 sq.-in.-blank or from a 125-mil thick by 121-sq.-in.-blank (SPE Journal, September 1972-Vol. 28, pages 38-42).
Further, in the September 1976 issue of Plastics World, page 53, there was a later announcement that a new grade of Azdel sheet based on PBT (polybutylene terephthalate) thermoplastic polyester reinforced with 30 weight percent of continuous glass fiber mat was being offered. These Azdel sheets were formed in a stamping operation after being preheated to 450.degree. F.-500.degree. F. (about 232.degree. C. to about 260.degree. C.).
Allied Chemical Corporation, for instance, produces a stampable nylon 6 composite sheet which is registered under the name STX, and has a combination of about 50% nylon 6 resin, about 30% glass fiber reinforcements and about 20% fillers. The composite sheet must be heated to a temperature above its melting point before it can be stamp formed, according to an article in the March 1979 issue of Plastics Engineering (pages 47-49).
In every instance mentioned above, the preheating of the composite sheet apparently has to take place at or above the melting point of the thermoplastic material being used in the sheet. Such heating, of course, requires a significant amount of energy that has to be used for each sheet. Also, heating at or above the melting point means that greater care must be exercised in transporting a sheet in its melt or above-melt state as from the infrared oven to the forming or stamping press.
One thermoplastic material that does not appear to be given as much mention in the literature for structural purposes as other thermoplastic materials is the polyester, poly(ethylene terephthalate). It is noted, for instance, in U.S. Pat. No. 3,547,891 that there is disclosed a thin film material or sheet material (about 7.5 to 10 mils in thickness) of poly(ethylene terephthalate), that has been vacuum heat formed, starting and ending essentially in the amorphous state. This amorphous final state would apparently be suitable for the final product, as for use in blister packages, as mentioned in the patent, but not for use in the final form of automobile parts. Another patent, U.S. Pat. No. 3,496,143, discloses a process for vacuum deep-drawing of poly(ethylene terephthalate) sheet material, which must have a solution viscosity [as determined in a 1% solution of the poly(ethylene terephthalate) in meta-cresol at 25.degree. C.] of about 1.7 to about 2.0 and a degree of crystallization of at least 5% up to about 25%. Neither this sheet material nor the one disclosed in U.S. Pat. No. 3,547,891 is a reinforced material or one of laminate construction. U.S. Pat. No. 3,496,143 specifies that its vacuum-formed product is not amorphous and that it has a higher degree of crystallinity than the initial material being molded; the molded material also being considered as having a degree of crystallinity in the range of 5% to 25%.
U.S. Pat. No. 3,765,998 discloses a high-impact resin sheet which is formable in shaping apparatus held at ambient temperature and concerns a glass mat having a glass fiber length of at least one inch, impregnated with poly(ethylene terephthalate) having a weight average molecular weight from about 5,000 to about 45,000. The sheets are preheated from about 240.degree. C. to about 280.degree. C., and are then transferred to a mold or press where they are cooled slowly under pressure to develop crystallinity (Examples 1 through 8). Examples 9 and 10 speak of chilling the laminate sheet, but since there is no indication of the rate of chilling taking place the state of crystallinity cannot be determined. In any event the patent teaches preheating to around the melt temperature for all examples. There is an indication in the specification that the "PET" [poly(ethylene terephthalate)] polymers have a level of crystallinity of from about 20% to about 60% as determined by X-ray techniques (column 3, lines 51-57), but it is not clear whether or not this statement has reference to the polymer in the pellet form prior to impregnation into the sheet or to the polymer when in the sheet form.
An advantage of the use of thermoplastic resin instead of the thermoset resin is that the former need only to cool below its crystallization temperature before a stamping press can be reopened and the part removed. A thermoset resin part must be given time for a chemical reaction to occur in order to cure the part before it can be removed from a stamping press.
Another advantage is that a thermoplastic part can be recycled, if need be, by reheating, whereas a thermoset part cannot be recycled by reheating.